(a) Field of the Invention
The present invention relates to a non-crosslinked metallocene catalyst having a novel structure for preparing polyethylene having high polymerizability, and a method of preparing the same.
(b) Description of the Related Art
A Ziegler-Natta catalyst which is widely applied in a conventional commercial process is a multi-site catalyst, and thus, since it is characterized in that a produced polymer has a broad molecular weight distribution, and the composition distribution of the comonomer is non-uniform, there is a limitation in securing desired physical properties.
However, a metallocene catalyst is a single-site catalyst having one kind of active point, and thus it has advantages in that the produced polymer has a narrow molecular weight distribution, and molecular weight, tacticity, crystallinity, in particular reactivity of a comonomer, may be significantly adjusted depending on the structures of a catalyst and a ligand.
As the metallocene catalyst, International Laid-Open Patent Publication No. 2008/084931 discloses a transition metal compound coordinated with a monocyclopentadienyl ligand to which an amido group is introduced. However, since a polyolefin polymerized by the metallocene catalyst according to the method has a narrow molecular weight distribution, when applied to some products there is a difficulty in field application, for example, productivity is significantly decreased due to an extrusion load and the like. Thus, in this regard, many efforts have been made to control the molecular weight distribution of a polyolefin. A polymer having a narrow molecular weight distribution has low melt flowability, low melt tension, and poor moldability, and may act as a factor for decreasing stiffness.
However, a metallocene catalyst precursor generally used in the preparation of a low molecular weight polyethylene has a small-sized ligand coordinated to a central metal, and has a structure in a non-crosslinked form. Though these catalysts may produce a low molecular weight polymer with high activity, they have a problem in that copolymerizability of a comonomer such as 1-hexene is relatively reduced by a small bite angle (Cp-Metal-Cp) as compared with other ansa-type catalysts.
In addition, in the case that a substituent is further introduced to the ligand, or bulk is increased, in order to implement a medium-low molecular weight based on supported polymerization, a molecular weight increase effect may be obtained, but it is a general trend to result in further decreased copolymerizability.
Therefore, there is need for development of a catalyst for preparing a polyethylene having excellent copolymerizability and a high molecular weight.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.